This invention relates to direct injection spark ignition engines and, more particularly, to a piston and combustion chamber for a reverse tumble direct injection engine of the spark ignition type.
Various types of direct injection spark ignition engines have been developed in efforts to provide improved fuel efficiency combined with acceptable control of exhaust emissions. Among these are engines utilizing stratified charge preparation involving horizontal cylinder swirl around a vertical axis, forward tumble involving vertical swirl from the combustion chamber center toward the outer edge around a horizontal axis, and reverse tumble involving cylinder swirl around a horizontal axis from the outer edge of the combustion chamber toward the center. While so called swirl type engines have had possibly the most development and interest, recent attention has also involved both forward and reverse tumble stratified charge preparation. Development of improved engines of the stratified charge type for use in future vehicle production appears desirable in order to meet continuing requirements for increased efficiency and emission control in the combustion process.
The present invention provides superior piston bowl and combustion chamber configurations for a reverse tumble spark ignition direct injection engine. The piston design includes a head end having a raised pedestal with dual shallow upwardly angled intake and exhaust faces intersecting at a linear peak. A shallow bowl is formed primarily in the intake face and extending from an outer side adjacent an edge of the head end of the piston to an inner side slightly beyond the peak.
The bowl features a bowl lip re-entrant angle at the spark plug location on the inner side of the bowl having an open or positive re-entrant angle in a range from about +10 to +15 degrees. On lateral edges of the bowl between the inner and outer edges, wing like extensions are provided which include negative re-entrant angles, preferably in a range from about xe2x88x9215 to xe2x88x9220 degrees. These are effective in containing the injected fuel within the bowl during combustion. Other features of the combustion chamber include vertical intake ports that produce a weak reverse tumbling air flow motion with a tumble ratio of about 0.6 which allows increased flow efficiency of the port.
An injector spray cone angle of 90 degrees is preferred which is combined with a bowl sized to optimally capture the 90 degree cone angle spray after accounting for the spray""s collapse at chamber back pressures typical of late injection timings for this type of engine. Too large a bowl width causes the mixture within the bowl to become too lean at the side extremes of the bowl and cause unburned hydrocarbon emissions to increase. Too narrow a piston bowl width causes part of the spray not to enter the bowl and be captured, which also causes an increase in hydrocarbon emissions. An injector mounted in the side of the cylinder head at an angle of about 47 degrees is presently preferred.
The piston provides a mixture preparation surface approaching the central spark plug location which features a large radius of curvature that promotes even spreading of the fuel in a 360 degree pattern after the fuel spray impacts on the surface of the bowl. It helps the soft spray, which has very low momentum to begin with, to travel on toward the spark plug and insure the existence of fuel air mixture at the spark plug gap at the time of ignition. Too small a radius of curvature decelerates the spray and compromises the ignition timing as well as the ignition process itself. Too large a radius of curvature causes a large fraction of the fuel vapor to escape beyond the spark plug gap into the squish region on the exhaust side of the chamber and results in poor ignition and unburned hydrocarbon emissions. Specific recommended dimensions and features for a preferred embodiment of the invention are provided and are discussed subsequently.